shell holes

with the mention of a skeleton I am assuming you are doing a diorama of an old battlefield?

in which case the skeleton could be someone who was KIA while taking cover in the impact crater.

 

I would suggest google Image searching WWI pictures and take a look at them. after a while out in the water they lose the rough edges and walls that are evident after a fresh blast. the rain and water smothens them out a bit. 

Hey, Ive built them before using sculpey, or any other kind of clay, on top of a styrofoam base, and self drying clay is easiest.  Then just paint diluted wood glue over and sprinkle some sand, paint your base, and give the inside of the crater a blast of black spraypaint to make it look scorched.  Heres a picture of one Im working on, made with this technique:

 

Hope this helps!

By the way, not trying to suggest you make it too gruesome, but I doubt a skeleton can get hit by a shell and stay in one piece, so you might want to scatter the bones a bit, or maybe even break a few.

Build up the base for the ground work with styrofoam and cut out a conical hole in it. Use your preferrred groundcover mixture over this. I like Cellucly mixed with white glue, brown acrylic paint and rough textured rmodel railroad turf and/or ballast along with water. Spread this over the styrofoam in a thin layer to cover everything. Build a small lip around the crater and before the Celluclay is set add some small pebbles randomly and any other things lilke tree stumps, barbed wire supports, broken equipment, etc. so it looks to be part of the landscape. Touch up any painting you need to the base work. Place your skeletons where you want them in the crater. Now mix up some two part epoxy based water, Liquitex, for example, and pour your first thin layer in th hole. When it dried, you might want to do a layer  of clear smoke over it or mix some clear red and green for a brown color. Let this dry and repeat he epoxy layer. A lighter coat of transucent paint and repeat until you get the depth you want. This may take a few days for the hole to be filled, since the epoxy takes about a day to cure. Your last layer should be clear. Check around the net, there are alos commercial tints for epoxy that you could use as well to eliminate the paint step. Paint the ground around the water darker than the rest.

I like to use celluclay over styrofoam for my groundscapes.  I would recommend carving a connical shaped crater in the styrofoam and texturing it with celluclay.  I do not know if the resin water products sold for Railroad layouts will be compatible with the styrofoam or celluclay.  I'd ask around.  Good luck with your WWI dio. 

Semper Fi,

Chris